This invention relates to electrolytic systems and more particularly to electrolytic cell design.
Many commercially important products are produced by electrolysis. For example, in the electrolysis of sodium chloride brine, chlorine gas is produced at the anode and caustic soda and hydrogen gas are produced at the cathode. In the electrolysis of water, oxygen gas is evolved at the anode while hydrogen gas is evolved at the cathode. In the refining of nickel, impure nickel is dissolved at the anode and pure nickel is deposited on the cathode. In electroorganic synthesis, organic compounds may be oxidized as in the Kolbe reaction, or reduced as in the hydro-dimerization of acrylonitrile to produce adiponitrile.
Electrolytic processes involve the forcing of a direct current through a suitable electrolyte between an anode and a cathode which are immersed in the electrolyte. The devices used to carry out such processes are known as electrolytic cells, the design of which must be adapted to the particular products being produced.
An electrolyte separator such as a diaphragm or membrane may be required in an electrolytic cell if the anolyte is necessarily of a different composition than the catholyte or if the anode and cathode products can mix. If the anode product can react with the cathode product, a separator can also prevent explosions.
Electrolytic cells which are adapted to evolve a product in the form of a gas must permit efficient removal of the gas from the electrolysis zone to prevent gas polarization. Therefore, in gas evolving cells, the electrodes are generally vertical.
In spite of the enormous variety of cell designs capital cost and unit energy consumption are factors that are considered in all cells. Low unit energy consumption, which results from a low cell voltage and a high current yield, or selectivity, is especially critical as the cost of power continues to increase. It is well known that the current efficiency of an electrolytic cell depends upon many factors, such as the nature of the electrodes, the nature and concentration of the electrolytes, and to some extent the current density. Power losses can result from cell overvoltages, as for example the excess voltage required to drive the anode reaction and the cathode reaction. These overvoltages depend upon the current density on the anode and cathode respectively, the temperature, and to some extent the concentrations of the electrolytes and the velocities of the electrolytes flowing over the electrodes.
Further power losses are the result of ohmic losses in the media between anode and cathode, as for example the IR.sub.a drop in the anolyte between anode and separator, the IR.sub.c drop in the catholyte between cathode and separator, and the IR.sub.d drop in the separator itself. The individual resistances R.sub.a, R.sub.c, R.sub.d, etc., depend partially upon the corresponding resistivities of the medium under operating conditions. It is obviously desirable to reduce the resistivity of the electrolyte to decrease power losses.
The volume of gas evolved at the electrode is proportional to the current supplied to the electrode. The accumulation of gas in the gap between electrode and separator, i.e., the electrolysis zone, increases the electrolyte resistance according to the equation: EQU (R/R.sub.o = [1-.epsilon.].sup.-3/2
where R is the electrolyte resistance with gas bubbles present, R.sub.o is the electrolyte resistance with no gas bubbles present, and .epsilon. is the volume fraction of gas present in the electrolyte. This increase in resistivity occurs because of the difficulty of electromigration of ions through a nonconductive gaseous matter.
In the several types of cells involving electrolysis of aqueous solutions, product gas bubbles rise through the liquid electrolyte carrying with them a liquid boundary layer. The bubbles burst after they emerge from the electrolyte, thereby releasing the product gas for collection. As the bubbles burst, the liquid boundary layer is released. This liquid then moves downward through the electrolysis zone to replace the liquid being brought up to the top of the cell on the boundary layers of other bubbles. The net result is a steady downward flow of liquid which opposes the upward movement of the bubbles, thereby reducing the velocity of the rising product bubbles. The concentration of bubbles present in the electrolyte therefore increases, increasing the electrolyte resistivity as previously discussed.
This increase in electrolyte resistivity can be minimized by enabling rapid escape of the bubbles from the electrolysis zone. One method of effecting this rapid escape is by an external electrolyte circulation system utilizing circulation pumps. These pumps can circulate electrolyte, which carries the gas product bubbles, upward through the electrolysis zone. However, the cost of the power to run the pump may cancel any power savings achieved by the lower electrolyte resistivity. Furthermore, higher construction and capital costs are required with an external circulation system, as for example for large external piping, additional floor space, large traps for separating product gases from electrolyte, and, in bipolar electrolyzers, for large currrent breakers to prevent current bypassing around the cells.
Other attempts have been made to enable a rapid escape of product bubbles. The electrodes of some electrolytic cells have vertically grooved surfaces with the ridges between the grooves of opposing electrodes used to support a flexible separator. With this design, it was attempted to provide a region for the evolved product bubbles to rise through the electrolysis zone without opposing the downward flow of electrolyte. In a cell of this type there is some tendency for the product gases to rise through the electrolyte near the electrolyte separator at the front of the grooves, and for the electrolyte to flow downward near the back of the grooves. However, the downward flow of electrolyte is counter to the rising product bubbles and any steady downward electrolyte flow is soon interrupted.